Gaseous discharge lamp starting circuits are well known in the art. Generally gaseous discharge lamps usually require a higher voltage for ignition than operation. This occurs since at ignition time, most cold gasses act as insulators, whereas during operation the hot vapors become conductive. As is known, the high voltage requirements for ignition of discharge lamps can be accomplished by appling high open circuit voltages at the line frequency across the discharge lamp or employing starting circuit which provide high voltage pulses to generate initial discharge in the lamp.
A number of high-pressure sodium discharge lamp lighting circuits have been developed in recent years such as that disclosed in U.S. Pat. No. 4,072,878 to Engel et al. The Engel et al patent discloses an apparatus that provides for high-voltage pulses for starting a sodium discharge lamp by using the breakdown characteristics of a Zener diode to provide accurately timed starting pulses. The use of the Zener diode eliminates timing problems encountered when the breakdown characteristics of a glow lamp are used in somewhat similar fashion, as described in U.S. Pat. Nos. 3,917,976 and 3,963,958 to Nuckolls.
Another apparatus for starting and operating a high-pressure sodium lamp is disclosed in U.S. Pat. No. 4,143,304 to Hitchcock et al. The Hitchcock et al apparatus uses a voltage amplification circuit, utilizing two individual capacitors, the output of which is applied across the ballast reactor which is connected to the reactor in an autotransformer relationship. Although the Hitchcock et al circuit works well, it does require a relatively large number of circuit components.
Further, various discharge lamp starting devices have been developed which employ non-linear dielectric elements. For example, U.S. Pat. No. 4,404,029 to Iwaya et al; U.S. Pat. No. 4,381,476 to Adachi et al; U.S. Pat. No. 4,399,390 to Oshita et al; U.S. Pat. No. 4,347,462 to Adachi, and U.S. Pat. No. 4,513,227 to Labadini et al. The references show many variations in the use of starting circuits and devices which utilize the pulse generating capabilities of non-linear elements. Another example of a starting device is shown in a West German patent application disclosure, D.E. No. 33-30-266-A1 of Iwasaki Electric Company in which a temperature sensitive switch, a semiconductor switch and a on-linear capacitor are mounted within the lamp envelope and connected in series across the electrodes of a lamp. The temeprature-sensitive switch is required to isolate the starting circuit from the power supply once the lamp has ignited.
While such prior art devices provide results in the areas intended, there still exists a need to provide a starting circuit for discharge lamps which employs inexpensive small components while providing a wide range of voltage pulses.
Accordingly, an object of the present invention is to provide an improved starting circuit for gaseous discharge lamps.
Another object of the invention is to provide a starting circuit which provides a range of pulse voltages for initiating discharge which is simple and economical in construction and reliable in operation.
Other objects and advantages of the present invention will become apparent from the following description and the appended claims.